Costume mask



FIG. 1 is a front perspective view of a costume mask showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a right side elevational view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a bottom plan view thereof; and

FIG. 6 is a top plan view thereof. 

The ornamental design for a costume mask, as shown. 